Chiller system with low capacity controller and method of operating same

ABSTRACT

A method ( 126 ) and a low capacity controller ( 28 ) enable a chiller system ( 20 ) to operate below a minimum allowable capacity. The chiller system ( 20 ) includes a chiller ( 40 ) and a chiller fluid loop ( 30 ), having a supply line ( 36 ) for conveying a chiller fluid ( 34 ) from the chiller ( 40 ) and a return line ( 38 ) for returning the chiller fluid ( 34 ) to the chiller ( 40 ). A capacity demand for the chiller ( 40 ) is determined. When the capacity demand is less than a minimum allowable capacity of the chiller ( 40 ), the chiller fluid ( 34 ) is routed from the return line ( 38 ) to a heat exchanger ( 96 ) of the low capacity controller ( 28 ) where the chiller fluid ( 34 ) is warmed and returned to the return line ( 38 ). The warmed chiller fluid ( 34 ) establishes a false capacity demand, detectable at the chiller ( 40 ), that is at least the minimum allowable capacity.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of chilled water systems. More specifically, the present invention relates to a method of operating a chilled water system.

BACKGROUND OF THE INVENTION

Chiller systems provide a temperature conditioned fluid, for use in conditioning the air within large buildings and other facilities. The chilled fluid is typically pumped to a number of remote heat-exchangers or system coils for cooling various rooms or areas within a building. A chiller system enables the centralization of the air conditioning requirements for a large building or complex of buildings by using water or a similar fluid as a safe and inexpensive temperature transport medium.

In general, a chiller of the chiller system provides chilled water of a particular temperature, via a first fluid loop, for cooling air in a building. Heat is extracted from the building air, transferred to the fluid in the first fluid loop, and is returned via the first fluid loop to the chiller. The returned fluid is again cooled to the desired temperature by transferring the heat of the fluid to the chiller's refrigerant. After the refrigerant is compressed by a compressor, the heat in the refrigerant is transported to the condenser and heat is transferred to a second fluid conveyed in a second fluid loop. The second fluid loop transports waste heat from the condenser of the chiller to a cooling tower which then transfers the waste heat from the second water loop to ambient air by direct contact between the ambient air and the second fluid of the second loop.

A multiple chiller system has two or more chillers connected by parallel or series piping to a common distribution system. Multiple chillers offer operational flexibility, standby capacity, and less disruptive maintenance. Through the use of multiple chillers, when the cooling demand is low, only one chiller may need to operate, and the operating chiller's capacity may be controlled to match the demand. If the cooling demand is beyond a single chiller's maximum capacity, one or more additional chillers may need to be activated. As such, the operating chillers are controlled so the system's total capacity (sum of the chillers' individual capacities) meets the cooling demand.

Chiller capacities normally are based on the maximum load anticipated. However, much of the time that a chiller is in use, it is operating at less than full load. Indeed, a chiller system in a building may operate over a wide range of demand conditions, with significant dominance of low capacity demand. Low capacity demand can result from seasonal fluctuations, when cooling a small office or zone during “off-hours”, when there is low or no load available for the start up of the chiller system, and so forth.

Low capacity demand typically causes a chiller to operate less efficiently. That is, lower capacity demand results in a higher kilowatt use per ton. More critically however, low capacity demand can force a chiller system to operate in unstable conditions. Under such conditions, compressor and evaporator capacities balance at ever lower suction pressures and temperatures. Typically, chillers are outfitted with protection mechanisms that cause them to shut down when the capacity demand becomes very low, for example, less than approximately eighteen percent of total chiller capacity. Left unchecked, the eventual result is coil frosting and compressor flooding.

Accordingly, some chillers are unable to operate when the capacity demand is very low. This is problematic for an operator who wishes to cool a small office or zone during “off-hours”, who needs to keep one office or room open past normal closing time, who has low or no load available for the start up of the chiller system, and so forth.

Attempts have been made to circumvent this problem by the inclusion of a hot gas bypass. A hot gas bypass can stabilize a chiller system by diverting hot, high-pressure refrigerant vapor from the discharge line directly to the low-pressure side of the chiller. This technique keeps the chiller compressor more fully loaded while the evaporator satisfies the part-load condition. In addition, the diverted vapor raises the suction temperature, which prevents frost from forming.

Although hot gas bypass can provide frost control and match system capacity to load to allow the system to operate at safe balance points during unsafe loads, hot gas bypass sometimes fails to safely stabilize the chiller system. Moreover, hot gas bypass can undermine the reliable operation of a chiller by introducing problems stemming from insufficient oil return and refrigerant logging in the hot gas bypass line. In addition, hot gas bypass requires an additional refrigerant line thus increasing the initial cost of a chiller system, and also increasing the likelihood of refrigerant leaks. Hot gas bypass further reduces operating efficiency because the bypassed vapor does no useful cooling.

Accordingly, what is needed is a method and apparatus for operating a chiller system below a minimum allowable capacity for the chiller system when a capacity demand is very low.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that a method and low capacity controller are provided for operating a chiller system.

It is another advantage of the present invention that a method and low capacity controller are provided that enable a chiller system to be operated below a minimum allowable capacity for the chiller system.

Yet another advantage of the present invention is that the low capacity controller can be readily and cost effectively incorporated into a chiller system.

The above and other advantages of the present invention are carried out in one form by a method of operating a chiller system, the chiller system including a chiller and a fluid loop, and the fluid loop including a supply line for conveying a fluid from the chiller and a return line for returning the fluid to the chiller. The method calls for determining a capacity demand for the chiller. When the capacity demand is less than a minimum allowable capacity of the chiller, the fluid is warmed in the return line to establish a false capacity demand, detectable at the chiller, that is at least the minimum allowable capacity.

The above and other advantages of the present invention are carried out in another form by a low capacity controller in a chiller system. The chiller system includes a chiller and a first fluid loop, the first fluid loop including a supply line for conveying a chiller fluid from the chiller and a return line for returning the chiller fluid to the chiller, the chiller being operable above a minimum allowable capacity for the chiller. The low capacity controller includes a heater, and a secondary loop interposed between the return line and the heater. The low capacity controller further includes means for enabling a transfer of the chiller fluid through the heater via the secondary loop when a capacity demand is less than the minimum allowable capacity, the chiller fluid being warmed at the heater, and means for returning the chiller fluid from the heater to the return line via the secondary loop.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, wherein like reference numbers refer to similar items throughout the Figures, and:

FIG. 1 shows a block diagram of a chiller system in accordance with a preferred embodiment of the present invention;

FIG. 2 shows a flowchart illustrating an operational process of the present invention in accordance with the preferred embodiment;

FIG. 3 shows a block diagram of the chiller system operating in a low capacity mode in accordance with the preferred embodiment;

FIG. 4 shows a block diagram of the chiller system operating in a chiller bypass/economizer mode;

FIG. 5 shows a block diagram of a low capacity controller for a chiller system in accordance with an alternative embodiment of the present invention; and

FIG. 6 shows a block diagram of a low capacity controller for a chiller system in accordance with another alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a chiller system 20 in accordance with a preferred embodiment of the present invention. In general, chiller system 20 includes a chiller fluid section 22, a refrigeration section 24, a condenser fluid section 26, a low capacity controller 28, and a system controller 29.

Chiller fluid section 22 includes a chiller fluid loop 30 and pumps 32. Pumps 32 are in fluid communication with chiller fluid loop 30 for forcing a chiller fluid, represented by arrow heads 34, to circulate within chiller fluid loop 30. Chiller fluid loop 30 includes a supply line 36 and a return line 38. Supply line 36 conveys chiller fluid 34 from chillers 40 of refrigeration section 24 to an air handler 42 for conditioning the air within a space served by air handler 42. Air handler 42 uses chiller fluid 34 to transfer heat energy from the air being circulated from the space by means of a fan 44 and ductwork (not illustrated) to a heat exchange coil 46 of chiller fluid loop 30. Return line 38 returns chiller fluid 34 to chillers 40, so that chiller fluid 34 can be subsequently re-cooled and re-circulated through chiller fluid loop 30.

Chiller fluid section 22 further includes one or more pump controllers or variable flow devices 48 for controlling a flow rate of chiller fluid 34 through chiller fluid loop 30 and a differential pressure sensor 50 for controlling a bypass valve 52 to maintain a minimum flow of chiller fluid 34 in chiller fluid loop 30. Chiller fluid section 22 further includes a flow meter 54, a supply temperature sensor 56, a return temperature sensor 58, and a return mixed temperature sensor 60 for staging chillers 40 on and off in response to a capacity demand (discussed below) and/or to control bypass valve 52 as known to those skilled in the art.

In a preferred embodiment, pumps 32 form a primary variable flow distribution system. Such a primary variable flow distribution system lowers initial costs, due to the elimination of the secondary pumps and associated fittings, vibration isolation, starters, and so forth, and uses less energy than conventional primary-secondary variable flow systems. However, the present invention may alternatively be adapted for use in a primary-secondary variable flow distribution system. In addition, only two pumps 32 are shown for simplicity of illustration. However it should be understood that chiller system 20 may include any number of pumps 32.

Although only one air handler 42 is illustrated herein, it should be readily apparent that chiller fluid loop 30 may distribute chiller fluid 34 to various system coils or heat exchangers for cooling rooms or other areas within a building.

Refrigeration section 24 includes one or more chillers 40. In this exemplary embodiment, chiller system 20 includes two chillers 40. However, the present invention may be adapted for use in chiller systems that include any number of chillers 40. In addition, chillers 40 need not be of the same capacity. As such, one of chillers 40 may have a higher maximum cooling capacity than the other.

In an exemplary embodiment, chillers 40 are screw compressors utilizing R134A refrigerant. Screw compressors are positive-displacement machines with nearly constant flow performance, and are usually available in units from about 30 to 1250 tons. A slide valve of a screw compressor is used to adjust the refrigerant's flow rate for varying the chiller's capacity or cooling effect. Although screw compressors are described herein, it should be apparent to those skilled in the art that other types of chillers may be employed as an alternative to screw compressors.

Generally, each of chillers 40 includes a compressor 62 that forces a refrigerant in series through a condenser 64, an expansion device 66 (for example, a flow restrictor, orifice, capillary, expansion valve, and so forth), and an evaporator 68 via a refrigeration conduit 70. Evaporator 68 conditions chiller fluid 34 to a predetermined temperature, for example, 45° F., so that chiller fluid 34 can be reused and conveyed in chiller fluid loop 30 to air handler 42. The energy extracted from chiller fluid 34 by evaporator 68 is transported by refrigeration conduit 70 to compressor 62 which lowers the condensation point of the refrigerant so that the refrigerant can be condensed by condenser 64.

Condenser fluid section 26 includes a cooling tower 72, pumps 74, and a condenser fluid loop 76 conveying a condenser fluid, generally represented by arrow heads 78. Second fluid loop 76 is interposed between cooling tower 72 and condenser 64 of each of chillers 40. When the refrigerant is condensed by condenser 64, the energy, in the form of heat is transferred to condenser fluid 78 in condenser fluid loop 76. Pumps 74 force the circulation of condenser fluid 78 through cooling tower 72 where the heat of condenser fluid 78 is transferred to ambient air.

Condenser fluid section 26 may further include one or more flow valves 80 and one or more check valves 82 associated with pumps 74 that regulate a flow rate and flow direction of condenser fluid 78 in condenser fluid loop 76. Condenser fluid section 26 may further include a differential pressure sensor 84 for controlling a bypass valve 86 to maintain a minimum flow of condenser fluid 78 in condenser fluid loop 76. In addition, a condenser fluid temperature sensor 88 and a condenser fluid flow meter 90 may be provided for monitoring the temperature and flow rate of condenser fluid 78 in condenser fluid loop 76.

As known to those skilled in the art, instead of operating energy intensive chillers during cool, dry outdoor conditions, the cooling effect of outside air may be utilized to provide direct cooling to a facility or process using cooling tower 72. This situation is sometimes known as “free cooling,” and the chillers are deactivated, i.e., the operation of chillers 44 is bypassed. Thus, a three-way valve 92 may be included in condenser fluid section 26 for selectively routing condenser fluid 78 through cooling tower 72, or for bypassing cooling tower 72. In addition, when chillers 44 are deactivated, condenser inlet valves 94 may be provided that close partially or entirely to limit or prevent a flow of condenser fluid 78 into condensers 64 when utilizing “free cooling.”

In accordance with a preferred embodiment of the present invention, chiller system 20 includes low capacity controller 28. Low capacity controller 28 advantageously enables chillers 44 to operate below a minimum allowable capacity for chillers 44. In general, the minimum allowable capacity may be approximately eighteen percent of total chiller capacity. Conventional chiller systems are outfitted with protection mechanisms that cause them to shut down when the capacity demand becomes very low, for example, less than approximately eighteen percent of total chiller capacity. Left unchecked, the eventual result is coil frosting and compressor flooding. Accordingly, conventional chiller systems cannot operate at very low capacity demands.

Low capacity controller 28 of the present invention enables chillers 44 to operate at a capacity demand much lower than eighteen percent of total capacity, for example, as low as approximately three percent of total capacity or lower. It will become readily apparent below, that low capacity controller 28 provides precise capacity control for these minimal capacity requirements. Minimum capacity requirements may occur when cooling a small office or zone during “off-hours” of “after-hours”, when there is low or no load available for the start up of the chiller system, at seasonal fluctuations, and so forth. Moreover, it will become readily apparent in the ensuing discussion that low capacity controller 28 may be further employed to take advantage of “free cooling,” by utilizing the cooling effect of outside air to provide direct cooling to a building.

Low capacity controller 28 includes a heater, in the form of a heat exchanger 96, and a secondary loop 98 interposed between chiller fluid section 22 and heat exchanger 96. In a preferred embodiment, secondary loop 98 includes an inlet conduit 100 in fluid communication with return line 38 for conveying a portion of chiller fluid 34 from return line 38 to heat exchanger 96.

Secondary loop 98 further includes an outlet conduit 102 having a first branch 104 selectively in fluid communication with supply line 36 and a second branch 106 selectively in fluid communication with supply line 36 of chiller fluid section 22. First means, in the form of a modulating valve 108, is positioned along first branch 104 of outlet conduit 102 for selectively returning the portion of chiller fluid 34 to return line 38. Modulating valve 108 enables a rate controlled flow of chiller fluid 34 to return line 38 (discussed below). Second means, in the form of a chiller side flow valve 110, is positioned along second branch 106 of outlet conduit 102 for selectively returning chiller fluid 34 to supply line 36 (discussed below).

Condenser fluid loop 76 includes a secondary loop section 112 interposed between condenser fluid section 26 and heat exchanger 96. More specifically, secondary loop section 112 includes an inlet section 114 in fluid communication with a condenser fluid supply line 116 of condenser fluid loop 76 and heat exchanger 96. Secondary loop section 112 further includes an outlet section 118 in fluid communication with heat exchanger 96 and a condenser fluid return line 120 of condenser fluid loop 76. Means, in the form of a condenser side flow valve 122, is positioned along outlet section 118 for selectively enabling a transfer of condenser fluid 78 from condenser fluid loop 76 through heat exchanger 96 and back to condenser fluid loop 76 (discussed below).

In an exemplary embodiment, heat exchanger 96 is a plate and frame heat exchanger that provides a safe, sanitary, and efficient method of transferring heat from one fluid medium to another as the two streams pass on opposing sides of intervening plates. In particular, heat is transferred between chiller fluid 34 and condenser fluid 78 in response to a particular mode of operation in which chiller system 20 is being operated. This heat transfer between chiller fluid 34 and condenser fluid 78 will be discussed in detail in connection with the flowchart of FIG. 2.

System controller 29 generally oversees, manages, and controls the various components of chiller system 20. System controller 29 may encompass a wide variety of electrical devices (programmable or not programmable) having the ability to provide various output signals in response to various input signals. This communication is schematically represented by a bi-directional arrow 124. Examples of system controller 29 include, but are not limited to, microcomputers, personal computers, dedicated electrical circuits having analog and/or digital components, programmable logic controllers, and various combinations thereof.

In addition, system controller 29 may be utilized to oversee and manage individual controllers of various equipment groups. In such a situation, cooling tower 72 may be associated with an individual controller, each of chillers 40 may be associated with an individual controller, and signals may be forwarded between the individual controllers and system controller 29. Such a controller is typically supplied from the manufacturer as a complete direct-digital-control (DDC) and monitoring package.

FIG. 2 shows a flowchart generally illustrating an operational process 126 of chiller system 20 in accordance with the preferred embodiment. In particular, system controller 29 (FIG. 1) may control chiller system 20 to provide the necessary capacity to satisfy a capacity demand, or load. Process 126 begins with a task 128.

At task 128, controller 29 determines the capacity demand. The capacity demand is a measure of the amount of cooling required, i.e., the total load, imposed upon chiller system 20 at a give instant. In a preferred embodiment, capacity demand is calculated as a function of the flow rate of chiller fluid 34 and the difference between the supply and return temperatures of chiller fluid 34, as follows: DEMAND (tons)=FLOW RATE*(CHWST−CHWMRT)500/12,000 where flow rate is detected at flow sensor 54 (FIG. 1), CHWST is the chilled water supply temperature measured at supply temperature sensor 56, and CHWMRT is the chilled water mixed return temperature measured at return mixed temperature sensor 60. Although the above function may be utilized to determine the capacity demand, those skilled in the art will recognize that alternative functions may be employed that provide a measure of the capacity demand.

Next, a task 130 is performed. At task 130, controller 29 determines the ambient conditions. Controller 26 can determine the ambient conditions by receiving signals that include, for example, outside air temperature, outside air temperature dew point, and the like, from the appropriate sensors, as known to those skilled in the art.

In response to data collection tasks 128 and 130, a query task 132 is performed. At query task 132, system controller 29 determines whether load requirements and ambient conditions are such that chiller system 20 may enter a chiller bypass mode, also known as an economizer mode. As discussed above, under cool, dry outdoor conditions, the cooling effect of outside air may be utilized to provide direct cooling using cooling tower 72 (FIG. 1). When ambient conditions indicate such is the case, process 126 proceeds to a task 134. At task 134, chiller system 20 (FIG. 1) is operated in a chiller bypass/economizer mode. Thereafter, process 126 loops back to tasks 128 and 130 to continue monitoring capacity demand and ambient conditions. The chiller bypass/economizer mode will be discussed in connection with a chiller system fluid flow schematic presented in FIG. 4.

However, when system controller 29 determines that load requirements and ambient conditions are such that chiller system 20 should not enter a chiller bypass mode, process flow proceeds to a query task 136. At query task 136, system controller 29 determines whether the capacity demand calculated at task 130, is less than a minimum allowable capacity. In a preferred embodiment, the capacity demand is compared with an internal table of system controller 29 which is the cooling capacity of the installed chillers 40. The internal table preferably contains a minimum allowable capacity figure for chiller system 20. Since the individual capacities for each of chillers 40 need not be the same, the minimum allowable capacity figure may be approximately eighteen percent of the total capacity of the lowest capacity one of chillers 40 in chiller system 20.

When the capacity demand is not less than the minimum allowable capacity, process flow proceeds to a task 138. At task 138, system controller 29 provides the signaling necessary to operate chiller system 20 in a nominal mode. For example, system controller 29 will signal a discrete output to energize the required number of chillers 40. To enable chillers 40, system controller 29 may generate the discrete outputs to energize variable flow devices 48, as needed. In addition, system controller 29 may generate discrete outputs to close condenser side flow valve 122, chiller side flow valve 110, and modulating valve 108 so that low capacity controller 28 is bypassed. Once one or more chillers 40 have been enabled, each chiller 40 may operate based on their stand alone product integrated controls. Thereafter, process 126 loops back to tasks 128 and 130 to continue monitoring capacity demand and ambient conditions.

When the capacity demand is less than the minimum allowable capacity, signifying a very low desired capacity, tasks 140, 142, and 144 are performed.

Referring to FIG. 3 in connection with tasks 140, 142, and 144, FIG. 3 shows a block diagram of chiller system 20 operating in a low capacity mode 146 in accordance with the preferred embodiment. FIG. 3 particularly illustrates fluid flow through low capacity controller 28 of chiller system 20. In order to clearly demonstrate the fluid flow, all of the reference numerals set forth in FIG. 1 are not repeated in FIG. 3. Rather, only those reference numerals that further the understanding of fluid flow in low capacity mode 146 are shown in FIG. 3. It should be understood, however, that the components of chiller system 20 in FIG. 1 are equivalent to and present in chiller system 20 illustrated in FIG. 3.

At task 140, system controller 29 provides the necessary signaling to route condenser fluid 78 to heat exchanger 96 via inlet section 114 of secondary loop section 112. This is accomplished by opening condenser side flow valve 122 which enables a flow of condenser fluid 78 through secondary loop section 112.

At task 142, system controller 29 provides the appropriate signaling to route a portion of chiller fluid 34, i.e., portion 34′, to heat exchanger 96 via inlet conduit 100 of secondary loop 98. Chiller side flow valve 110 of secondary loop 98 is maintained in a closed position. Accordingly, portion 34′ of chiller fluid 34 is routed through heat exchanger 96 and returned to supply line 36 via first branch 104 of outlet conduit 102. Portion 34′ of chiller fluid 34 mixes with chiller fluid 34 supplied from chillers 40, as denoted in FIG. 3 by the numerals 34+34′. Since chiller system 20 is operating at low capacity, much of this mixed chiller fluid 34+34′ is returned to return line 38 via bypass valve 52 to maintain a minimum flow of mixed chiller fluid 34+34′ in chiller fluid loop 30.

At heat exchanger 96, heat is transferred from the warmer condenser fluid 78 to the cooler portion 34′ of chiller fluid 34. This causes the temperature of portion 34′ of chiller fluid 34 to increase. Consequently, portion 34′ causes the chiller fluid in return line 38, i.e., mixed chiller fluid 34+34′, of chiller fluid loop 30 to be warmer than what was originally measured at return mixed temperature sensor 60. This warmed chiller fluid 34+34′ establishes a “false capacity demand” from the perspective of chillers 40, so that chillers 40 will not enter into an automatic chiller shutdown due to a freeze protection condition.

Low capacity mode 146 may be most clearly explained by example. Under normal conditions, let it be assumed that the supply and return differential temperature of chiller fluid 34 is 16° F. when operating at minimum flow and full capacity. Under this condition with a design supply temperature of 45° F., the temperature of returning chiller fluid 34 should be approximately 61° F. If the minimum allowable capacity of chillers 40 is eighteen percent, then the minimum allowed return temperature is approximately 48° F. Anything below this temperature is too cold for chiller 40. Because chiller 40 cannot download any further, chiller fluid 34 leaving chillers 40 drops below the set point of 48° F. Without the inclusion of the present invention, this “too cold” condition can cause the initiation of a freeze alarm, and cause chillers 40 to shut down. By warming chiller fluid 34 in return line 38, the temperature of chiller fluid 34 can be kept above this alarm condition, for example, above 52° F.

Modulating valve 108 is adjusted, i.e., modulated, to allow enough flow of portion 34′ of chiller fluid 34 into return line 38. This flow is tied to the capacity demand formula presented above. That is, the temperature of chiller fluid 34 is warmed so that the false capacity demand detectable at chillers 40 is above the minimum allowable capacity.

Following tasks 140, 142, and 144 operational process 126 loops back to tasks 128 and 130 to continue monitoring capacity demand and ambient conditions. Chiller system 20 remains in low capacity mode 146 until the capacity demand, as determined at task 128, becomes greater than the minimum allowable capacity, or unless chiller system 20 is able to enter the chiller bypass/economizer mode as determined through ambient conditions at task 130.

FIG. 4 shows a block diagram of chiller system 20 operating in a chiller bypass/economizer mode 148 in response to task 134 (FIG. 2) of process 126 (FIG. 2). FIG. 4 particularly illustrates fluid flow through low capacity controller 28 of chiller system 20 when in chiller bypass/economizer mode 148. In order to clearly demonstrate the fluid flow, all of the reference numerals set forth in FIG. 1 are not repeated in FIG. 4. Rather, only those reference numerals that further the understanding of fluid flow in chiller bypass/economizer mode 148 are shown in FIG. 4. It should be understood, however, that the components of chiller system 20 in FIG. 1 are equivalent to and present in chiller system 20 illustrated in FIG. 4.

In chiller bypass/economizer mode 148, system controller 29 provides the necessary signaling to route condenser fluid 78 to heat exchanger 96 via inlet section 114 of secondary loop section 112. This is accomplished by opening condenser side flow valve 122 which enables a flow of condenser fluid 78 through secondary loop section 112. An “X” positioned over each portion of condenser fluid loop 76 entering condensers 64 represents no- or minimal-flow conditions of condenser fluid 78 into condensers 64 since chillers 40 may be deactivated in chiller bypass/economizer mode 148. This may be accomplished by partially or entirely closing condenser inlet valves 94.

In addition, system controller 29 provides the appropriate signaling to route chiller fluid 34 to heat exchanger 96 from return line 38 via inlet conduit 100 of secondary loop 98 and back to supply line 36 of chiller fluid loop 30. This is accomplished by opening chiller side flow valve 110 and maintaining modulating valve 108 in a closed position. Accordingly, chiller fluid 34 is routed through heat exchanger 96 and returned to supply line 36 via second branch 106 of outlet conduit 102. An “X” positioned over supply line 36 directed away from chillers 40 and return line 38 directed toward chillers 40 represents no- or minimal-flow conditions of chiller fluid 34 into evaporators 68 since chillers 40 may be deactivated in chiller bypass/economizer mode 148.

At heat exchanger 96, heat is transferred from the warmer chiller fluid 34 to the cooler condenser fluid 78. This causes the temperature of chiller fluid 34 to decrease. This cooled chiller fluid 34 is subsequently routed to air handler 42 to cool the space served by air handler 42. Chiller bypass mode 148 is sometimes referred to as an economizer, “waterside”, or “free cooling” mode because cooling is achieved utilizing the cooling effect of outside air to provide direct cooling instead of operating the energy intensive chillers 40. Accordingly, low capacity controller 28 achieves the secondary benefit of providing direct cooling, as well as enabling chillers 40 to operate under very low load conditions by establishing a false capacity demand, as discussed above.

FIG. 5 shows a block diagram of a low capacity controller 150 for chiller system 20 in accordance with an alternative embodiment of the present invention. Low capacity controller 150 simply replaces low capacity controller 28 described in detail above. Accordingly, all of the reference numerals and components set forth in FIG. 1 are not repeated in FIG. 5. Rather, only those reference numerals and components that further the understanding of low capacity controller 150 are shown in FIG. 5.

Low capacity controller 150 includes heat exchanger 96, and a secondary loop 152 interposed between chiller fluid section 22 (FIG. 1) and heat exchanger 96. In this alternative embodiment, secondary loop 152 includes inlet conduit 100 in fluid communication with return line 38 for conveying portion 34′ of chiller fluid 34 from return line 38 to heat exchanger 96.

Secondary loop 152 further includes an outlet conduit 154 having a first branch 156 selectively in fluid communication with return line 38 and second branch 106 selectively in fluid communication with supply line 36 of chiller fluid section 22. Like low capacity controller 28 (FIG. 1), low capacity controller 150 also includes chiller side flow valve 110 positioned along second branch 106 of outlet conduit 102 for selectively returning chiller fluid 34 to supply line 36 when chiller system 20 (FIG. 1) operates in the economizer mode (as discussed above).

A pump 158 is placed in series with modulating valve 108 along first branch 156 of outlet conduit 154 for selectively returning portion 34′ of chiller fluid 34 directly to return line 38. Due to the higher flow rate and volume of chiller fluid 34 in return line 38, pump 158 is utilized to force the flow of portion 34′ of chiller toward return line 38, while modulating valve 108 enables the rate controlled flow of chiller fluid 34 to return line 38. Accordingly, portion 34′ of chiller fluid 34 and returning chiller fluid 34 are mixed in return line 38, as denoted by reference numerals 34+34′, when chiller system 20 (FIG. 1) operates in low capacity mode 146 (FIG. 3).

As discussed above, heat is transferred from the warmer condenser fluid 78 to the cooler portion 34′ of chiller fluid 34. This causes the temperature of portion 34′ of chiller fluid 34 to increase. Consequently, portion 34′ causes the chiller fluid in return line 38, i.e., mixed chiller fluid 34+34′, of chiller fluid loop 30 to establish the “false capacity demand” from the perspective of chillers 40, so that chillers 40 will not enter into an automatic chiller shutdown due to a freeze protection condition.

Like low capacity controller 28 (FIG. 1), low capacity controller 150 also includes chiller side flow valve 110 positioned along second branch 106 of outlet conduit 154 for selectively returning chiller fluid 34 to supply line 36 when chiller system 20 (FIG. 1) operates in the economizer mode (as discussed above in connection with FIG. 4).

FIG. 6 shows a block diagram of a low capacity controller 160 for chiller system 20 (FIG. 1) in accordance with another alternative embodiment of the present invention. Low capacity controller 160 simply replaces low capacity controller 28 described in detail above. Accordingly, all of the reference numerals and components set forth in FIG. 1 are not repeated in FIG. 6. Rather, only those reference numerals and components that further the understanding of low capacity controller 160 are shown in FIG. 6.

Low capacity controller 160 includes heat exchanger 96, and a secondary loop 162 interposed between chiller fluid section 22 (FIG. 1) and heat exchanger 96. In this alternative embodiment, secondary loop 162 includes inlet conduit 100 in fluid communication with return line 38 for conveying portion 34′ of chiller fluid 34 from return line 38 to heat exchanger 96.

Secondary loop 162 further includes an outlet conduit 164 having a first branch 166 selectively in fluid communication with return line 38 and second branch 106 selectively in fluid communication with supply line 36 of chiller fluid section 22 (FIG. 1). A three-way valve 168 controls the flow of warmed portion 34′ of chiller fluid 34 into return line 38, thus, its subsequent mixing with returning chiller fluid 34 in return line 38, while modulating valve 108 enables the rate controlled flow of chiller fluid 34 to return line 38 (as discussed above. Accordingly, portion 34′ of chiller fluid 34 and returning chiller fluid 34 are mixed in return line 38, as denoted by reference numerals 34+34′.

Like low capacity controllers 28 (FIG. 1) and 150 (FIG. 5), low capacity controller 160 also includes chiller side flow valve 110 positioned along second branch 106 of outlet conduit 164 for selectively returning chiller fluid 34 to supply line 36 when chiller system 20 (FIG. 1) operates in the economizer mode (as discussed above in connection with FIG. 4).

In summary, the present invention teaches of a method and low capacity controller for operating a chiller system. The low capacity controller enables a chiller system to be operated below a minimum allowable capacity for the chiller system by warming chiller fluid prior to its return to the chillers. This establishes a false capacity demand at the chillers to prevent the chillers from going into an alarm and shut-down condition. In addition, the low capacity controller achieves a secondary benefit through its use in a chiller bypass/economizer mode to provide direct cooling through the utilization of the cooling effect of outside air. The low capacity controller is readily and cost effectively incorporated into a chiller system through the use of a plate and frame heat exchanger, and an extension of the fluid loops to route the warmed chiller fluid back to the return line when in the low capacity mode and to route the cooled chiller fluid back to the supply line when in the chiller bypass/economizer mode.

Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims. For example, the utilization of the heat exchanger in the chiller bypass/economizer mode may be optional. As such, the present invention may be adapted to include a heat exchanger that utilizes a fluid other than the condenser fluid. 

1. A method of operating a chiller system, said chiller system including a chiller and a fluid loop, said fluid loop including a supply line for conveying a fluid from said chiller and a return line for returning said fluid to said chiller, said method comprising: determining a capacity demand for said chiller; and when said capacity demand is less than a minimum allowable capacity of said chiller, warming said fluid in said return line to establish a false capacity demand, detectable at said chiller, that is at least said minimum allowable capacity.
 2. A method as claimed in claim 1 wherein said determining operation comprises: measuring a supply temperature of said fluid in said supply line; measuring a return temperature of said fluid in said return line; and ascertaining a difference between said return temperature and said supply temperature to determine said capacity demand.
 3. A method as claimed in claim 2 wherein said determining operation further comprises calculating said capacity demand as a function of a flow rate of said fluid and said difference between said return temperature and said supply temperature.
 4. A method as claimed in claim 1 wherein said chiller system further includes a heat exchanger in communication with said return line, said fluid is a first fluid, and said warming activity includes transferring heat from a second fluid in said heat exchanger to said first fluid.
 5. A method as claimed in claim 4 wherein said second fluid is condenser fluid, and said method further comprises transporting said condenser fluid to said heat exchanger via a condenser fluid loop interposed between said chiller and a cooling tower of said chiller system.
 6. A method as claimed in claim 1 wherein said fluid is a first fluid, and said method further comprises: routing a portion of said first fluid in said return line through a secondary loop, said secondary loop being in heat exchanging relation with a second fluid; and returning said portion of said first fluid back to said return line via said secondary loop.
 7. A method as claimed in claim 6 wherein said returning activity comprises modulating a return of said portion of said first fluid to said return line.
 8. A method as claimed in claim 6 wherein said secondary loop is coupled with said supply line, and said returning operation comprises: conveying said portion of said first fluid into said supply line; and routing said portion of said first fluid to said return line via a bypass valve of said chiller system interposed between said supply line and said return line.
 9. A method as claimed in claim 6 wherein said secondary loop is coupled with said return line, and said returning operation comprises routing said portion of said first fluid from said secondary loop directly into said return line.
 10. A method as claimed in claim 9 further comprising regulating a flow of said portion of said first fluid into said return line.
 11. A method as claimed in claim 6 further comprising: selectively operating said chiller system in a chiller bypass mode; when in said chiller bypass mode, deactivating said chiller; when in said chiller bypass mode, transporting said portion of said first fluid through said secondary loop to cool said portion of said first fluid; and when in said chiller bypass mode, returning said portion of said first fluid from said secondary loop to said supply line.
 12. A method as claimed in claim 1 wherein said warming activity comprises adjusting a volume of said fluid warmed in response to said capacity demand.
 13. A method as claimed in claim 1 further comprising when said capacity demand is greater than said minimum allowable capacity, discontinuing said warming activity.
 14. A chiller system for providing a fluid to a heat exchange unit comprising: a chiller; a pump for forcing said fluid through said chiller; a fluid loop having a supply line for conveying said fluid from said chiller to said heat exchange unit, and a return line for returning said fluid from said heat exchange unit to said chiller; means for determining a capacity demand for said chiller; and when said capacity demand is less than a minimum allowable capacity of said chiller, means for warming said fluid in said return line to establish a false capacity demand, detectable at said chiller, that is at least said minimum allowable capacity.
 15. A chiller system as claimed in claim 14 wherein said determining means comprises: a first temperature sensor for measuring a supply temperature of said fluid in said supply line; a second temperature sensor for measuring a return temperature of said fluid in said return line; means for determining a flow rate of said fluid in said fluid loop; and means for calculating said capacity demand as a function of said flow rate and a difference between said return temperature and said supply temperature.
 16. A chiller system as claimed in claim 14 wherein said warming means comprises: a heat exchanger; and a secondary loop in communication with said return line and in heat exchanging relation with said heat exchanger, a portion of said fluid in said return line being routed through said secondary loop to warm said fluid in said heat exchanger when said capacity demand is less than said minimum allowable capacity, and said portion of said fluid being returned to said return loop via said secondary loop when said fluid is warmed.
 17. A chiller system as claimed in claim 16 wherein: said chiller system further comprises a bypass valve interposed between said supply line and said return line; and said secondary loop comprises: a supply conduit for transporting said portion of said fluid from said return line toward said heat exchanger; a return conduit coupled with said supply line for conveying said portion of said fluid from said heat exchanger; and a modulating valve in fluid communication with said return conduit for modulating a return of said portion of said fluid to said return line, said portion of said fluid being returned to said return line via said bypass valve.
 18. A chiller system as claimed in claim 16 wherein: said secondary loop comprises: a supply conduit for transporting said portion of said fluid from said return line toward said heat exchanger; a return conduit coupled with said return line for returning said portion of said fluid from said heat exchanger to said return line; a modulating valve in fluid communication with said return conduit for modulating a return of said portion of said fluid to said return line; and said chiller system further comprises means for regulating a flow of said portion of said first fluid into said return line.
 19. A chiller system as claimed in claim 18 wherein said regulating means is a pump interposed along said return conduit.
 20. A chiller system as claimed in claim 18 wherein said regulating means is a three-way valve positioned at a junction of said return conduit with said return line.
 21. A chiller system as claimed in claim 16 further comprising: a cooling tower; and a condenser fluid loop interposed between said chiller and said cooling tower, said second fluid loop being routed through said heat exchanger, and said condenser fluid loop conveying a second fluid through said heat exchanger when said capacity demand is less than said minimum allowable capacity such that heat from said second fluid is transferred to said portion of said fluid.
 22. A chiller system as claimed in claim 21 further comprising: means for selectively operating said system in a chiller bypass mode; means for, in said chiller bypass mode, deactivating said chiller; means for, in said chiller bypass mode, transporting said portion of said fluid through said secondary loop for enabling heat transfer in said heat exchanger between said condenser fluid loop and said secondary loop to cool said portion of said fluid; and means for, in said chiller bypass mode mode, returning said portion of said fluid from said secondary loop to said supply line.
 23. A low capacity controller for a chiller system, said chiller system including a chiller and a first fluid loop, said first fluid loop including a supply line for conveying a chiller fluid from said chiller and a return line for returning said chiller fluid to said chiller, said chiller being operable above a minimum allowable capacity for said chiller, and said low capacity controller comprising: a heater; a secondary loop interposed between said first fluid loop and said heater; means for enabling a transfer of said chiller fluid through said heater via said secondary loop when a capacity demand is less than said minimum allowable capacity, said chiller fluid being warmed at said heater; and means for returning said chiller fluid from said heater to said return line via said secondary loop.
 24. A low capacity controller as claimed in claim 23 wherein said chiller system further includes a cooling tower and a condenser fluid loop interposed between said chiller and said cooling tower, said condenser fluid loop carrying a condenser fluid, and said low capacity controller further comprises means for enabling a transfer of said condenser fluid through said heater via said condenser fluid loop when said capacity demand is less than said minimum allowable capacity for said chiller.
 25. A low capacity controller as claimed in claim 23 wherein said returning means comprises a modulating valve for modulating a return of said chiller fluid to said return line.
 26. A low capacity controller as claimed in claim 23 wherein said chiller system includes a bypass valve interposed between said supply line and said return line, and said secondary loop comprises: a supply conduit for transporting said portion of said fluid from said return line toward said heat exchanger; a return conduit coupled with said supply line for conveying said portion of said fluid from said heat exchanger, said portion of said fluid being returned to said return line via said bypass valve.
 27. A low capacity controller as claimed in claim 23 wherein said returning means adjusts a volume of said chiller fluid returned to said return line in response to said capacity demand. 